Complement resistance is an important virulence factor in the pathogenesis of bacterial gram negative infections. A more complete understanding of complement resistance determinants may lead to improves prevention or treatment of infections caused by these organisms. We have used transposon mutagenesis to randomly mutate a clinical isolate of Escherichia coli that is highly serum resistant. Isogenic mutants have been detected that have an increased sensitivity to serum. Of these, one group is capsule minus. We have identified that at least three physical loci are involved in capsule production and they are linked. One of the capsule genes disrupted has DNA homology to the Neisseria capsular transport gene. Capsule also protects against killing by neutrophils, but not the bactericidal effects of the defensins. In a systemic mouse model of infection capsule minus strains cause significantly less mortality. Therefore, these results strongly suggest that capsule is a major virulence factor. Studies on the regulation of capsule synthesis demonstrate increased gene transcription as growth temperature increases from 22 degrees celsius to 42 degrees celsius. Other isogenic mutants, however, are capsule positive but serum sensitive. This demonstrates tat bacterial serum resistance is multi-factorial process.